This invention relates to a process and apparatus for the low temperature fractionation of air and, in particular, to a system which can accommodate a variable oxygen demand.
In various branches of industry, oxygen demand is subject to relatively large fluctuations in time intervals of minutes, hours, or days. From the standpoint of process control, the inertia of an industrial-scale, low temperature air fractionating column and associated apparatus is so high that it is uneconomical, in response to short-term demand changes, to manipulate the flow rate of the air feed which would result in an upset in the steady state design conditions of the column. Any such upset would also have deleterious effects on the efficiency of the separating process.
Conversely, it is just as disadvantageous to store excess oxygen in pressurized gas tanks and then withdraw such oxygen upon increased demand. Expensive, large pressurized gas tanks and additional compression energy would be necessary for this purpose.
For these reasons, a process has been developed for flexible oxygen production wherein fractionation products are withdrawn from the rectification column in the liquid phase and stored in liquid holding tanks. Such a process, with one tank each for oxygen and nitrogen, is known, for example, from Linde Reports on Science and Technology, No. 37/1984, pp. 18-20.
In the previously published process, liquid oxygen from the oxygen tank is fed into the bottom of the low pressure stage during the time period when a larger amount of gaseous oxygen is needed than can be produced by the column based on the amount of air introduced. This liquid oxygen is vaporized in the bottom of the low pressure stage in heat exchange with pressurized nitrogen at the head of the high pressure stage. Nitrogen is liquefied during the heat exchange, withdrawn from the high pressure stage, and stored in the nitrogen tank. During periods when excess gaseous oxygen is obtained, the stored liquid nitrogen becomes available as reflux for the low pressure column. This extra reflux thereby provides excess oxygen which is withdrawn in the liquid phase from the bottom of the low pressure column and stored in the oxygen tank.
In the conventional process with alternating storage by means of two liquid holding tanks, the amount of fractionated air remains constant at all times. In this method, a steady state operation of the rectification is obtained in the high pressure stage as well as in the low pressure stage.
In case of increased oxygen demand, it is necessary to have sufficient gaseous nitrogen available at the head of the high pressure stage so as to vaporize liquid oxygen in the bottom of the low pressure stage, permitting the withdrawal of such oxygen as a gaseous product. For this reason, under a normal load, a certain excess amount of gaseous high pressure nitrogen must be withdrawn in order to be able to maintain constant column separation rates. This amount of high pressure nitrogen removed during normal load operation is then available in case of increased oxygen demand for the vaporization of oxygen. However, this amount of nitrogen does not affect the rectification since during high load operation, both liquefied nitrogen from the head of the high pressure column and vaporizing oxygen at the bottom of the low pressure column are immediately withdrawn and do not participate in the mass transfer and heat transfer operations in the column. Thus, during high load operation, excess nitrogen is stored as liquid nitrogen in the nitrogen tank, while vaporized oxygen is withdrawn as the desired product.
During the period of high oxygen demand, the quantity of additional oxygen that can be withdrawn, i.e., the fluctuation range of the product quantity, is, in effect, determined by the amount of high pressure nitrogen removed in the gaseous phase during normal load. This portion of the nitrogen produced in the high pressure stage basically is not introduced into the low pressure stage but rather is removed from the process, either directly as a gaseous product (in the normal load case and in case of lowered oxygen demand) or through intermediate storage in the nitrogen tank (in case of increased oxygen demand). Therefore, independently of the load presently involved in the operation, this amount of nitrogen is not available as reflux for the low pressure column.
This lack of reflux has an adverse effect on the degree of rectification in the low pressure stage, which is especially deleterious if it is desired to produce a side stream of argon. For the latter purpose, a tap is made in the low pressure stage at a point of increased argon concentration, the so-called argon bulge. The extent of this argon bulge depends, however, greatly on the reflux ratio. The argon concentration at this point, and thus the possible argon yield as well, decrease if less than the entire amount of nitrogen produced in the high pressure stage is introduced in the liquid phase into the low pressure stage. For this reason, the rectifying relationships in the low pressure column and specifically the argon yield are unsatisfactory in the prior art process for variable oxygen production, and the severity of this product is increased as the fluctuation range of the oxygen product is increased.